The present invention relates to a sprocket for a roller chain. FIG. 1 shows both a chain C having rollers R engaging sprockets S1 and S2. In particular, the present invention relates to a sprocket that reduces the noise generated when the roller chain and sprocket mesh.
Noise generated by a roller chain meshing with a sprocket is a long recognized problem. One of the most significant sources of noise in a typical roller chain drive results from the repeated impacts between the rollers and the sprockets as the rollers seat between the teeth of the sprocket. As shown in FIG. 2, a roller 10 engaging a sprocket S swings on an arc 20 having a radius equal to the chain pitch P around the center of a seated adjacent roller 30. The roller 10 swings along the arc to make a substantially radial impact with the sprocket tooth root 40.
The prior art discloses attempts to reduce the noise created by rollers impacting a sprocket. One such attempt to reduce the noise levels generated by roller and sprocket collisions is shown in U.S. Pat. No. 6,030,306. This patent discloses a technique that staggers the chain meshing with multiple sprockets such that the overall noise level generated by the multiple sprocket collisions is reduced. However, this method does not avoid substantially radial collisions or reduce the noise generated by collisions between the rollers and individual sprocket tooth roots.
Other sprocket designs have attempted to decrease the impact noise of roller chain drives by altering the sprocket tooth profile. An early example of such a design is disclosed by U.S. Pat. No. 3,448,629. This patent describes modifying the bottom diameter of the sprockets, widening the gap between adjacent teeth of the sprockets, and attaching one or more rubber rings near the outer periphery of each sprocket.
A more recent attempt to reduce roller chain and sprocket noise using a modified sprocket tooth profile is described by U.S. Pat. No. 5,921,877. A sprocket tooth profile having three sections is described as making the roller of a chain come into engagement with the teeth of the sprocket without generating a large noise. However, this design is limited to a unidirectional roller chain drive because of the asymmetry of the tooth profile as generally shown by FIG. 3.
Other designs also utilized an asymmetrical tooth design in an effort to reduce the noise generated by the impact of the rollers as they strike the sprocket. U.S. Pat. No. 5,876,295 discloses an asymmetrical tooth profile with flank radii that may differ from the drive side to the coast side of the tooth. U.S. Pat. Nos. 5,921,878 and 5,993,344 disclose an asymmetrical tooth profile that incorporates a flank flat that is tangential to an engaging flank radius and a first root radius, and a pitch mismatch wherein the sprocket chordal pitch is less than the chain chordal pitch. This arrangement is described as facilitating a staged roller tooth contact with the sprocket. Again, the asymmetry of the tooth profile mandates a unidirectional roller chain drive system.
Other asymmetrical tooth designs, such as that disclosed by U.S. Pat. No. 5,997,424, provide a tooth space clearance between the roller and a portion of the tooth root in an attempt to reduce the noise generated by the vibrational contact of unseated rollers with the sprocket teeth as the rollers wear. FIG. 4 illustrates one such design. Instead of impacting the bottom of the tooth root, the roller makes radial contact with the flanks of adjacent teeth at locations A1 and A2 while bridging over the bottom of the tooth root. This sprocket is also designed for a unidirectional roller chain drive system. Similarly, U.S. Pat. No. 5,397,278 simply reduces the gap between adjacent sprocket teeth such that the roller makes simultaneous radial contact with the flanks of adjacent teeth. However, this design does allow for use in a multi-directional roller chain drive system.
There remains a long-standing need for a multi-directional sprocket that substantially minimizes the noise generated by the substantially radial impacts between the rollers of a roller chain and a sprocket. The present invention addresses this need as well as others previous disadvantages of the prior art.
A sprocket according to the present invention is a multi-directional roller chain sprocket that minimizes substantially radial roller impact with the sprocket. This sprocket provides a tooth root-to-chain pitch relationship that causes a substantially tangential impact between the roller and the sprocket tooth flank to smoothly and quietly transmit the connecting energy of the chain and sprocket. A related feature of a sprocket according to the present invention is distribution of the connecting energy of the roller and sprocket over time to avoid impact energy related noise.
In one embodiment, a known roller chain is selected to match the pitch of the sprocket. The roller chain has a plurality of links, interconnected by pivot pins. A roller member that is freely rotatable about the pivot pin is mounted on each pivot pin. The roller members have a cylindrical outer surface and are characterized by a tolerance stack that represents the variation of the diameter of the outer cylindrical surface between the largest and smallest diameters of roller members. The roller chain is constructed to engage a sprocket having a plurality of teeth spaced about its periphery. The sprocket has a pitch circle passing through the center of each roller member when those members are fully engaged with the sprocket.
The sprocket defines generally circular roots between pairs of adjacent teeth. Each of the roots is defined by a tooth root radius, which is the distance from a point on the root to a point on the sprocket pitch circle centered between the teeth adjacent to the root. The tooth root radius is determined to minimize substantially radial impacts of the roller members with the sprocket. The roller members engage the sprocket teeth on their flanks in a glancing, or substantially tangential fashion. The sprocket may have tooth root radii equal to the roller radius, the radius of the outer cylindrical surface of the roller, plus the tolerance stack between the roller members and the chordal rise and fall of the chain. However, a lesser tooth root radius may suffice.